Spam in Gmail "All Mail" in Mail.app

I have a Gmail account configured in Mail.app in OS X 10.5. I have never had any problems with it except that lots of spam shows up in my "All Mail" folder within that account, but that spam does not show up in the Gmail Inbox within Mail.app. It seems that my Gmail Spam folder is catching some spam, but is still leaving a lot in the All Mail folder. I have tried deleting the flagged spam in All Mail, but for some reason it shows right back up after I delete it. My two questions are: What do I need to do to successfully delete the flagged spam in my All Mail folder? And, how can I set up a rule to move all of the flagged spam in All Mail to my junk mail folder (either through Gmail webapp or Mail.app)? Thanks in advance...

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